Asthma is a disease that makes it difficult to breathe and in many cases can be debilitating. Asthma is generally manifested by (i) bronchoconstriction, (ii) excessive mucus production, and/or (iii) inflammation and swelling of airways that cause widespread but variable airflow obstructions. Asthma can be a chronic disorder often characterized by persistent airway inflammation, but asthma can be further characterized by acute episodes of additional airway narrowing via contraction of hyper-responsive airway smooth muscle tissue.
Conventional pharmacological approaches for managing asthma include: (i) administering anti-inflammatories and long-acting bronchodilators for long-term control, and/or (ii) administering short-acting bronchodilators for management of acute episodes. Both of these pharmacological approaches generally require repeated use of the prescribed drugs at regular intervals throughout long periods of time. However, high doses of corticosteroid anti-inflammatory drugs can have serious side effects that require careful management, and some patients are resistant to steroid treatment even at high doses. As such, effective patient compliance with pharmacologic management and avoiding stimuli that triggers asthma are common barriers to successfully managing asthma.
Asthmatx, Inc. has developed new asthma treatments that involve applying energy to alter properties of the smooth muscle tissue or other tissue (e.g., nerves, mucus glands, epithelium, blood vessels, etc.) of airways in a lung of a patient. Several embodiments of methods and apparatus related to such treatments are disclosed in commonly-assigned U.S. Pat. Nos. 6,411,852, 6,634,363, 7,027,869, and 7,104,987; and U.S. Published Application Nos. US2005/0010270 and US2006/0247746, all of which are incorporated by reference herein in their entirety.
Many embodiments of the foregoing asthma treatments that apply energy to tissue of the airways use catheters that can be passed (e.g., navigated) through the tortuous passageways defined by the lung airways. FIG. 1, for example, illustrates a bronchial tree 90 in which the various bronchioles 92 decrease in size and have many branches 96 as they extend from the right and left bronchi 94. Accordingly, the treatment devices should be configured to treat airways of varying sizes as well as function properly when repeatedly deployed after navigating through the tortuous anatomy.
It is also desirable to control the amount and rate of energy delivered to the treatment site. For example, the energy delivery devices for delivering radio frequency (RF) energy to tissue in the lung airways disclosed in the commonly-assigned patents and applications incorporated by reference above have been controlled by measuring the temperature of one of the electrodes during energy delivery. Other types of treatment devices that deliver RF energy for other applications outside of the lung airways, such as ablation and cauterization devices, have controlled the delivery of energy to cardiac and vasculature tissue based on measuring factors other than temperature. For example, ablation and cauterization devices have monitored impedance during a procedure and terminated energy deliver when a sharp increase in the impedance is measured. This sharp increase may correlate with a desired end result, such as tissue desiccation or protein denaturation. As such, existing ablation and cauterization systems may terminate energy delivery based on a direct relationship between an increase in impedance and an increase in temperature.